In the medical field, a doctor displays, on a monitor, a medical image obtained by sensing an object and diagnostically interprets the displayed medical image, thereby observing the state or time-rate change of a morbid portion. Many of the medical images are tomographic images inside the object. Examples of medical image collection apparatuses (modalities) for obtaining a tomographic image are an ultrasonic image diagnosis apparatus, magnetic resonance imaging apparatus (MRI), X-ray computerized tomography apparatus (X-ray CT), and optical coherence tomography (OCT).
It is difficult to grasp the state of an unclear image sensing region existing in an obtained tomographic image just by observing individual tomographic images (obtained tomographic images) obtained by these modalities.
The unclear image sensing region will be described here with reference to FIG. 17. The unclear image sensing region is an unclear region in the image sensing region of an obtained tomographic image where the luminance is higher or lower than usual. For example, when an ultrasonic image diagnosis apparatus is used as the medical image collection apparatus for obtaining a tomographic image, a shadow region or a posterior echo region corresponds to the unclear image sensing region. The shadow region is a region where the state inside the object is not visualized in the obtained tomographic image because the ultrasonic probe serving as an image sensing unit is not in appropriate contact with the object surface. Note that if the ultrasonic probe is not in contact with the object surface at all, the entire image sensing region of the obtained tomographic image is detected as the shadow region. The posterior echo region is an unclear region in the image sensing region of an obtained tomographic image where the luminance is higher or lower than usual because of the influence of a tumor or the like inside the object. Note that in the following description, the outside of the image sensing region of an obtained tomographic image is not included in the unclear image sensing region.
Non-patent reference 1 (W. Wein, A. Khamene, D-A. Clevert, O. Kutter, and N. Navab, “Simulation and fully automatic multimodal registration of medical ultrasound,” Proc. MICCAI'07, vol. 1, pp. 136-143, 2007.) discloses a method of obtaining a tomographic image using a given modality, generating an image of the same slice based on three-dimensional geometric volume data acquired by another modality, and displaying both images in a superimposed manner. According to this method, even if the obtained tomographic image contains an unclear image sensing region, it is possible to grasp the state of the unclear image sensing region as far as a corresponding region is visualized in the tomographic image (generated tomographic image) generated based on the three-dimensional geometric volume data acquired by another modality. Patent reference 1 (Japanese Patent No. 3871747) discloses a method of displaying an obtained tomographic image and a generated tomographic image side by side. This method also allows to obtain the same effect as non-patent reference 1.
It is difficult to grasp the three-dimensional shape or range of a morbid portion only by observing individual tomographic images obtained by the above-described modalities. There is an attempt to reconstruct three-dimensional shape data from a tomographic image group. If the three-dimensional shape data has been reconstructed, analysis or display is done based on the reconstructed data, thereby easily grasping the three-dimensional shape or range of a morbid portion.
A medical image collection apparatus such as an MRI, X-ray CT, or OCT obtains a tomographic image group at an equal interval. It is therefore possible to easily reconstruct three-dimensional shape data by simply stacking the tomographic images. However, an ultrasonic image diagnosis apparatus normally performs image sensing while a doctor or a technician holds an ultrasonic probe in hand and freely moves it. For this reason, the position of the space based on the human body, which is visualized in each obtained tomographic image, is unknown. An attempt has been made to measure the position and orientation of the ultrasonic probe using an external sensor and obtain the positional relationship between tomographic images, thereby reconstructing three-dimensional shape data (non-patent reference 2 (A. Fenster, “3-Dimensional Ultrasound Imaging,” Imaging Economics, 2004)). Another attempt has also been made to estimate the positional relationship between tomographic images based on the correlation between image features in the tomographic images without using any external sensor, thereby reconstructing three-dimensional shape data (non-patent reference 3 (T. A. Tuthill, J. F. Krucker, J. B. Fowlkes, and P. L. Carson, “Automated three-dimensional US frame positioning computed from elevational speckle decorrelation,” Radiology, vol. 209, pp. 575-582, 1998.)).
The tomographic image group is obtained while pressing the probe against the object. For this reason, the object deforms due to the pressure of the probe, and the reconstructed three-dimensional shape data is distorted.
To prevent this, a deformation-free object shape is acquired separately, and deformation is corrected based on it. For example, a method disclosed in non-patent reference 4 (W. Wein, B. Roper, and N. Navab, “Automatic registration and fusion of ultrasound with CT for radiotherapy,” Proc. MICCAI 2005, vol. 2, pp. 303-311, 2005.) acquires the deformation-free three-dimensional shape data of an object in advance using X-ray CT, and generates a simulated ultrasonic image based on it. A tomographic image actually obtained by an ultrasonic image diagnosis apparatus is associated with the simulated image based on image information. This enables correction of deformation.
Still another attempt has been made to estimate and correct the object deformation amount caused by the probe pressure. For example, non-patent reference 5 (G. M. Treece, R. W. Prager, A. H. Gee, and L. Berman, “Correction of probe pressure artifacts in freehand 3D ultrasound,” Medical Image Analysis, vol. 6, no. 3, pp. 199-214, 2002.) discloses a method of estimating and correcting, based on an image feature in a tomographic image and the measured value of a position and orientation sensor, the amount of deformation caused by the probe pressure. This method assumes that deformation occurs only in the probe pressure application direction. The deformation amount corresponding to the depth from the body surface of each tomographic image is estimated and corrected by calculating the correlation between the horizontal lines of tomographic images at adjacent image sensing times. At this time, the correction is done by adjusting the estimated value of the deformation amount of each tomographic image such that the estimated values of the deformation amounts and the measured values of the position and orientation sensor at the image sensing start and end times are consistent.
However, in the method of non-patent reference 1, a clearly sensed region (clear image sensing region) and an unclear image sensing region in an obtained tomographic image are not explicitly discriminated. Hence, the generated tomographic image covers the clear image sensing region as well and impedes observation of the clear image sensing region. Even if, for example, a biopsy cannula is included in the clear image sensing region, the cannula is hard to view.
In the method of patent reference 1, the doctor needs to complement the unclear image sensing region in the obtained tomographic image by observing the corresponding region in the generated tomographic image. At this time, it is impossible to accurately determine the correspondence between the unclear image sensing region in the obtained tomographic image and a region in the generated tomographic image. This still makes it difficult to grasp the state of the unclear image sensing region.
The method of non-patent reference 4 independently requires a medical image collection apparatus such as X-ray CT to obtain the reference object shape without deformation caused by the probe pressure. That is, it is impossible to reconstruct a three-dimensional shape without deformation from only a tomographic image group obtained by an ultrasonic image diagnosis apparatus.
In the method of non-patent reference 5, the measured value obtained by the position and orientation sensor represents the position and orientation of the deformed body surface, and the correct value of the amount of deformation caused by the probe pressure cannot be found. This makes it impossible to accurately correct the deformation and obtain the reference object shape without deformation.